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Published 27 October 2003. doi:10.1083/jcb.200306020
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© The Rockefeller University Press, 0021-9525/2003/10/257 $8.00
The Journal of Cell Biology, Volume 163, Number 2, 257-269

Article

 Formation of stacked ER cisternae by low affinity protein interactions

Erik L. Snapp1, Ramanujan S. Hegde1, Maura Francolini2, Francesca Lombardo2, Sara Colombo3, Emanuela Pedrazzini4, Nica Borgese2,5 and Jennifer Lippincott-Schwartz1

1 Cell Biology and Metabolism Branch, National Institutes of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892
2 Consiglio Nazionale delle Ricerche Institute of Neuroscience, Cellular and Molecular Pharmacology Section
3 Department of Medical Pharmacology, University of Milan, 20129 Milano, Italy
4 Consiglio Nazionale delle Ricerche Istituto Biologia e Biotecnologia Agraria, 20133 Milano, Italy
5 Department of Pharmacobiology, University of Catanzaro, 88021 Roccelletta di Borgia, Catanzaro, Italy

Address correspondence to Jennifer Lippincott-Schwartz, Cell Biology and Metabolism Branch, National Institutes of Child Health and Human Development, National Institutes of Health, 18 Library Dr., Bldg. 18T, Rm. 101, Bethesda, MD 20892. Tel.: (301) 402-1010. Fax: (301) 402-0078. email: jlippin{at}helix.nih.gov

The endoplasmic reticulum (ER) can transform from a network of branching tubules into stacked membrane arrays (termed organized smooth ER [OSER]) in response to elevated levels of specific resident proteins, such as cytochrome b(5). Here, we have tagged OSER-inducing proteins with green fluorescent protein (GFP) to study OSER biogenesis and dynamics in living cells. Overexpression of these proteins induced formation of karmellae, whorls, and crystalloid OSER structures. Photobleaching experiments revealed that OSER-inducing proteins were highly mobile within OSER structures and could exchange between OSER structures and surrounding reticular ER. This indicated that binding interactions between proteins on apposing stacked membranes of OSER structures were not of high affinity. Addition of GFP, which undergoes low affinity, antiparallel dimerization, to the cytoplasmic domains of non–OSER-inducing resident ER proteins was sufficient to induce OSER structures when overexpressed, but addition of a nondimerizing GFP variant was not. These results point to a molecular mechanism for OSER biogenesis that involves weak homotypic interactions between cytoplasmic domains of proteins. This mechanism may underlie the formation of other stacked membrane structures within cells.

Key Words: endoplasmic reticulum; photobleaching; cytochrome b5; GFP; FRAP

Abbreviations used in this paper: b(5), cytochrome b(5); b(5) tail, truncated cytochrome b(5) containing amino acids 94–134; C1(1-29)P450, truncated cytochrome P450 containing amino acids 1–29; Deff, effective diffusion coefficient; IP3R, inositol 1,4,5-trisphosphate receptor; mGFP, monomeric GFP; NE, nuclear envelope; OSER, organized smooth ER; TMD, transmembrane domain.


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